The present invention relates generally to a nuclear magnetic resonance (MR) imaging method and apparatus that picks up nuclear magnetic resonance signals from hydrogen and phosphorus in a patient and transforms the distribution of density of nuclei and the distribution of relaxation time of nuclear spins into images, more particularly to MR imaging method and apparatus that reduce motion artifacts.
When a patient moves during MR image acquisition, great artifacts occur on a reconstructed image. The artifacts are called motion artifacts, and occur because a phase of a collected echo signal is shifted by the patient motion and echo signals including the echo signal with the phase shift is Fourier transformed in a phase-encoding (or slice-encoding) direction. Thus, when the patient moves even a few millimeters, the whole image blurs in the encoding direction and it is a great impediment to the clinical diagnosis as a result.
To reduce the artifacts, a method in which navigator echoes are used has been introduced (S.-G. Kim et al., xe2x80x9cFast Interleaved Echo-Planar Imaging with Navigator: High Resolution Anatomic and Functional Images at 4 Teslaxe2x80x9d, Magnetic Resonance in Medicine 35, 1996, pp. 895-902). In this method, in the sequence shown in FIG. 6, a gradient magnetic field pulse that generates a navigator echo is applied between the application of a radiofrequency (RF) pulse 201 and a routine 211, and the navigator echo is thereby generated and collected. Each time the RF pulse 201 is applied, at least one navigator echo is collected. Then, the navigator echo collected within a certain repetition time 209 is a reference, and the patient motion is presumed from the difference between the reference navigator echo and the navigator echoes collected within a repetition time other than the certain repetition time 209. The phase shifts of echo signals (image echo signals) used for image reconstruction are corrected on the basis of the presumed patient motion. At this time, the patient motion is slow and a repetition time 210 is short, and thus the patient is regarded as motionless within one repetition time 210.
The following methods are known in which the phase shifts of the image echo signals are corrected with the navigation echoes: in a method (a), each navigator echo is Fourier transformed to find the phase shift from a correlation, and the image echo signals are corrected according to the phase shift; and in a method (b), each navigator echo is Fourier transformed to find a difference in phase between the navigator echoes, and a phase shift of data in hybrid space acquired by Fourier transforming the image echo signals is corrected in real space according to the phase shift of the corresponding navigator echo (R. L. Ehman et al., xe2x80x9cAdaptive Technique for High-Definition MR Imaging of Moving Structuresxe2x80x9d, Radiology 173, 1989, pp. 255-263).
However, in the method (a), a long calculation time is needed for Fourier transformation, and the precision is low since the phase shift of the navigator echo is found from the correlation. In the method (b), effects of only small patient motions that are some times as large as 0.1 pixel can be corrected.
When the MR imaging is applied to IVR (Interventional Radiology) in which a technique such as biopsy or catheter insertion is performed simultaneously with the imaging, it is required to correct effects of a patient motion of 5 mm or so in realtime. However, it can not be realized in the conventional methods described above.
The present invention has as its object the provision of MR imaging method and apparatus that can acquire images without motion artifacts by correcting phase shifts of image echo signals that are caused by a patient motion of more than some pixels in realtime.
To achieve the above-mentioned object, according to the present invention, in an MR imaging method such as multi-shot EPI method and 3D-EPI method where a segment in which at least one image echo signal is generated each time one RF pulse is applied is repeated to acquire data required for reconstruction of one image; at least one navigator echo is generated each time the RF pulse is applied. Then, a phase shift of each navigator echo is found in k-space with one of the navigator echoes being a reference, and phase shifts of M image echo signals are corrected in k-space according to the phase shifts of corresponding navigator echoes.
An MR imaging method according to the present invention is a characterized in that: in the MR imaging method for acquiring image data required for reconstruction of an image by repeating a plurality of times a segment in which an RF pulse and gradient magnetic fields in three directions that are perpendicular to each other are applied to a patient to time-sequentially generate and collect image echo signals, the MR imaging method comprises the steps of: (a) generating and collecting at least one navigator echo in each of the plurality of segments; (b) producing in k-space a phase shift map of each navigator echo collected in each of the plurality of segments with the navigator echo collected in a certain segment among the plurality of segments being a reference; and (c) correcting in k-space a phase of the image echo signals collected in each of the plurality of segments according to each phase shift map of each navigator echo collected in each of the plurality of segments.
An MR imaging method according to the present invention is characterized in that: the MR imaging method imaging method comprises the steps of: (a) applying a gradient magnetic field in a slicing direction to a patient to select a slice substantially at the same time as applying an RF pulse; (b) applying a gradient magnetic field pulse in xcex1-direction that is one of the slicing direction, a phase-encoding direction and a readout direction to generate and collect a navigator echo; (c) applying a phase-encoding gradient magnetic field pulse which gives an offset for a phase-encoding and a readout gradient magnetic field pulse which gives an offset in the readout direction; (d) applying readout gradient magnetic field pulses of successively alternating polarity and applying phase-encoding gradient magnetic field pulses in synchronization with the readout gradient magnetic field pulses to time-sequentially generate and collect image echo signals within each cycle of the alternating readout gradient magnetic field pulses; (e) repeating a plurality of times a segment comprising the steps (a)-(d); (f) producing, in k-space as a function in the xcex1-direction, a phase shift map of each navigator echo collected in each of the plurality of segments with the navigator echo collected in a certain segment among the plurality of segments being a reference; (g) correcting in k-space a phase at each time phase of the image echo signals collected in each of the plurality of segments according to each phase shift map of each navigator echo collected in each of the plurality of segments; and (h) reconstructing an image from the corrected image echo signals.
Preferably, a phase of each phase shift map may be fitted with a function in the step (f), or a phase of each phase shift map may be filtered in the step (f).
According to the MR imaging method of the present invention, the phase difference between the navigator echoes is found in k-space as a function in the xcex1-direction (for example kx-direction), and hence Fourier transformation and correlation processing are not needed. Therefore, the phase shifts of the image echo signals caused by the patient motion can be precisely corrected at a high speed. Thus, the motion artifacts can be removed almost perfectly.